logging: init logging early, hand ProxyFetchFactory correct server log

- Prevent logging to stdout/stderr, make sure we log to error.log for
early messages during initialization. Note that nginx is still working
to setup its logging configuration, so these early messages will go
through its defaults. Which means that only warnings or worse will pass
for early logging messages.
- Make sure we init NgxServerContext's NgxMessageHandler to the correct
server{} - specific log so it will write to the error_log configured
in the server{} block instead of the global error_log.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/808
Helps https://github.com/pagespeed/ngx_pagespeed/issues/817